fix(qss): Set menu background color

As a black text color is enforced for the menu, we should also enforce a
white background for a good contrast.
This is more or less a workaround as this does not respect any system
style settings (dark mode, ...).
This fix will become obsolete once tackling #138 (removing/reducing stylesheet).

Fixes #139

src/blackgui/share/qss/stdwidget.qss

@@ -489,6 +489,7 @@ BlackGui--CDockWidgetInfoArea > QMenu > QMenuWidget {
 /* reset menu style */
 QMenu {
   color: black;
+  background-color: white;
   font-style: normal;
   font-weight: normal;
   padding: 5px;